Worm gears are one of the most commonly applied gear types in industrial applications. They can be found on heavy equipment and machinery where the high reduction ratios are required. Though the operating efficiency is considered lower than other gear configurations, the high reduction ratio and self-locking characteristics makes worm gears irreplaceable in certain applications.
A worm gear set is constructed of a worm and a gear. The gear is often referred to as a worm wheel. Regardless of the name, it is essentially a helical gear that matches the pitch, pressure angle, and helix angle of the mating worm. The worm and the worm gear are positioned perpendicularly. The shafts of these two components are oriented 90 degrees to each other.
The worm has its presentation almost like a screw except that there is a V type thread on a worm. The worm is the part that serves as the driving component in a worm gear set. When the worm rotates, the thread advances the teeth of the worm gear and drives the worm gear to rotate accordingly. The thread on a worm is called start. A worm can have one single start; it may have multiple starts as well.
For a single-start worm, a full turn of the worm advances the worm gear by one tooth. Take a worm gear with 24 teeth for example, the gear with 24 teeth provides a gear reduction ratio of 24:1. As for a multi-start worm, the gear reduction equals the number of teeth on the gear divided by the number of starts on the worm.
The speed ratio is usually referred to as the reduction ratio. The reduction ratio of a worm gear set is determined by the number of teeth on the worm gear and the number of thread starts on the worm. To be exact, the reduction ratio is the ratio of the number of teeth to the number of thread starts. A worm with a single thread has a very high reduction ratio. The ratio can be changed by replacing the worm gear set.
The worm gear set is a friction drive, which means the thread repeatedly engages with the teeth of the worm gear, and the material of the worm has to be harder than that of the worm gear as a result. The worm is usually made from steel or steel-based materials which have higher strength and corrosion resistance and the worm gear is typically made from bronze or aluminum alloys. When the worm gear pair is used under high load or operating at high speeds, such as inside a reduction gearbox, the worm may be made from hardened steel.
A worm gear pair is considered with a relatively lower efficiency than other gear set types because of the action of the worm gear. The meshing or the worm and the gear is a mixture of sliding and rolling actions in nature. The sliding action dominates at high reduction ratios and results in friction and heat. The friction and heat limits the efficiency of a worm gear pair profoundly.
Though the sliding contact of a worm gear pair reduces the efficiency, the sliding action yet provides the gear set with quiet operation capability. This feature makes the worm gear pair very ideal for applications where noise is undesirable and sought to be minimized. For example, elevators are the application where worm gear sets are primarily implemented.
The other key feature of a worm gear pair is the self-locking mechanism. Self-locking of worm gear refers to the working principle that the worm gear can be advanced and driven by the worm while the worm cannot be driven by the worm gear. Theoretically speaking, a worm gear pair is self-locking when the coefficient of friction between the gear and the worm is greater than the tangent of the worm’s lead angle. Though not always true, a typical worm gear pair is most of the time self-locking. The self-locking mechanism is useful in applications where lifting and holding loads are involved.
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